The present invention relates to a printing apparatus which is capable of printing a gray scale and, more particularly to a printing apparatus in which a thermal transfer film of an electrothermal type has the capability of providing a gray scale. The printing apparatus of the invention can deliver printed matter such as a color reproduction or picture.
At the present state of development of the information art, display devices which deliver picture information are being improved to be capable of color reproduction. Several difficult technologies are proposed as a basis of such color printing. Such technologies include methods such as ink jet, thermal transfer and wire dot printing. Although these methods have some disadvantages, it is possible to adapt them for use in apparatus for the printing of several colors (about seven to sixteen colors) and such methods have been put into practical use. However, in printing apparatus such as these, it is difficult to print a picture image of a color television display and to provide an economical and dependable printing apparatus having the requisite high printing speed and quality. This is because each of the aforesaid methods use a two-level recording process. In some circumstanses, a three or four-level recording method is also possible in which one dot is printed or is not printed. When representing a gray scale, therefore, it is necessary to use a signal process, such as a dither method or a matrix area gray scale method, in which several dots are combined into one unit. These methods increase cost and decrease printing speed and quality.